Your accounting ignores the uncertainty inherent in immigrant labor -- i.e. all the indirect costs associated with severe labor shortages when the political winds shift. The story in Georgia a few years back is telling [1]. For example, if immigration is curtailed, labor shortages will cause issues. If immigration is permitted, then healthcare costs could skyrocket. Robots are (at least) predictable -- and that can be a huge win in a lot of cases.

[1] http://www.forbes.com/sites/realspin/2012/05/17/the-law-of-u...